The bundles of plates for this type of heat exchanger generally include a stack of plates parallel to each other and which delimit between them circuits for circulating at least two independent fluids.
The circulation of the fluids between the plates may be of a convergent type, with crossed currents or with a counter current and each circuit is connected to admission and return collectors for the fluids.
The plates of the bundle of plates are equipped with different means with which the heat exchange co-efficient between the fluids may be increased.
For this, bundles of planar plates are known, between which are interposed fins made by folding thin metal sheets and assembled by brazing on the plates.
For high temperature applications, for example above 600° C., the use of a metal filler and of thin fins may pose problems.
Indeed, the metal filler having a lower melting point than the base metal, the temperature of use of the bundle of plates is limited and the use of a metal filler, the thermomechanical properties of which are different from those of the base material generate a weak point at the junction. Further, as the fins are very thin, for example, with a thickness of less than 0.5 mm, the enlargement of the grains during the brazing may prove to be redhibitory from a mechanical point of view.
A bundle of plates is also known, consisting of a stack of plates in which channels are engraved.
The engravement is achieved by chemical etching of the plates after positioning a protective mask on the portions not to be etched. Once they are etched, the plates are assembled by welding and notably by diffusion welding.
Making a bundle of plates with this technique is complex to apply and poses problems.
Chemical etching of the plates with nickel-based alloys is very difficult and manufacturers use mechanical machining techniques which are long and costly and do not leave much geometrical freedom for forming channels on the plates.
Moreover, for diffusion welding of nickel-based alloys, it is necessary to strongly heat the material. Indeed, whether this is an embodiment by hot isostatic compression or by axial uniform compression, the plates to be assembled are brought to a temperature above 1000° C. so that this heat treatment leads to enlargement of the grains, incompatible with the small thicknesses of the metal sheets.
The bundle of plates obtained by this technique has great stiffness because the plates are welded together in the whole exchange area and for this reason it is not very tolerant to thermal transients.
Another drawback of these two types of bundles of plates, lies in the fact that the welded or brazed junctions are made in the whole exchange area between the channels which are of small size which makes the inspection of these joints impossible, whether during manufacturing or during use of the bundle of plates.
A bundle of plates comprising a stack of metal heat exchange plates parallel to each other including edges with a smooth surface and a central portion provided with corrugations is also known for example from FR-A-2 738 906, for forming with the associated plates a dual circuit of two independent fluids and with a counter-current.
The assembling of the plates of this type of bundle of plates is achieved either by welding by means of a set of spacers positioned on the edges of the plates in order to obtain the required space between the plates, or by welding an added part acting as a spacer, itself welded on the longitudinal edge of each plate, and then welded together.